James Graham wrote:
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> I think there is more to it than the answers seem to indicate . . .
> A DC 5Kw motor puts out the rated power at the rated RPM's and the
> 10Kw motor puts out its rated power at its rated RPM's. If the design
> or the two motors RPM's is the same then how do you "throttle back" to
> only get 5 Kw from the bigger motor? Does the "throttle back" mean
> the RPM's are decreased? That would affect the propeller output
> ability. What exactly does the "power module" do when the DC motor is
> throttled back? Is the input voltage reduced to get lower thrust at
> the propeller? Are the RPM's held constant while the energy put into
> the motor is reduced? Sounds like a lot of more complicated answers
> as to the nature of how a DC motor's power output is varied.
>
> --- On *Sun, 11/8/09, Colin Girvan /<colingirvan@
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For my DC permanent magnet motor.
The RPM is dependant on the voltage. The controller (via a PWM circuit)
varies the voltage. You put 24v across the motor it will spin at a
certain RPM. With no load this will pull very little current. The 'kw'
(Volts X Amps) will be very small.
Connect the drive shaft with the propeller - the RPM will stay the same
(more or less, particularly if the motor isn't overloaded), but the Amps
will be MUCH higher. Now the volts which controls the speed is the same
but the motor is pulling more Amps to maintain it. Go back to the Power
= Volts X Amps sum. Bigger number on the power side.
In theory the more you 'load' the motor, the more Amps it will pull in
an effort to maintain the RPM. No motor is this perfect, so this isn't
really true, but it's close enough for 'getting your head around' the idea.
Of course there are other factors that stop a motor from having
infinite power if you could pour enough amps in, besides the high
currents melting things. Motors are only designed to carry a certain
amount of power - particular current limits based on the size of the
wire in the windings.
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